Electrochemical sensors are well known. It has also been proposed in the prior art to provide a diamond based electrochemical sensor. Diamond can be doped with boron to form semi-conductive or fully metallic conductive material for use as an electrode. Diamond is also hard, inert, and has a very wide potential window making it a very desirable material for use as a sensing electrode for an electrochemical cell, particularly in harsh chemical, physical, and/or thermal environments which would degrade standard metal based electrochemical sensors. In addition, it is known that the surface of a boron doped diamond electrode may be functionalized to sense certain species in a solution adjacent the electrode.
One problem with using diamond in such applications is that diamond material is inherently difficult to manufacture and form into suitable geometries for sophisticated electrochemical analysis. To date, diamond electrodes utilized as sensing electrodes in an electrochemical cell have tended to be reasonably simple in construction and mostly comprise the use of a single piece of boron doped diamond configured to sense one physical parameter or chemical species at any one time. More complex arrangements have involved introducing one or more channels into a piece of boron doped diamond through which a solution can flow for performing electrochemical analysis. However, due to the inherent difficulties involved in manufacturing and forming diamond into multi-structural components, even apparently relatively simple target structures can represent a significant technical challenge.
In terms of prior art arrangements, WO 2005012894 describes a microelectrode comprising a diamond layer formed from electrically non-conducting diamond and containing one or more pin-like projections of electrically conducting diamond extending at least partially through the layer of non-conducting diamond and presenting areas of electrically conducting diamond at a front sensing surface. In contrast, WO2007107844 describes a microelectrode array comprising a body of diamond material including alternating layers of electrically conducting and electrically non-conducting diamond material and passages extending through the body of diamond material. In use, fluid flows through the passages and the electrically conducting layers present ring-shaped electrode surfaces within the passages in the body of diamond material.
More recently, it has been proposed that high aspect ratio boron doped diamond electrodes have improved sensing capability when compared with other boron doped diamond electrode arrangements. That is, it has been found to be highly advantageous to provide boron doped diamond electrodes which have a high length/width ratio at a sensing surface. Furthermore, it has been found that an array of high aspect ratio boron doped diamond electrodes providing a band sensor structure can be utilized to provide multiple sensing functions.
The previously described arrangements may comprise optically opaque, electrically conductive boron doped diamond electrodes spaced apart by optically transparent, non-conductive intrinsic diamond layers. The optically opaque, electrically conductive boron doped diamond electrodes can be driven to perform electrochemical measurements of species in aqueous solution. It has also been suggested that electrochemical techniques can also be combined with optical techniques such as spectroscopic measurements by using the non-conductive intrinsic diamond layers as an optical window as described in WO2007/107844. As such, electrochemical measurements can be performed at the optically opaque, electrically conductive boron doped diamond electrodes and optical measurements of the solution can be performed through non-conductive intrinsic diamond layers.
Swain et al. describe a combined electrochemistry-transmission spectroscopy technique for analysing chemical species in solution. The technique uses an electrochemical cell comprising an optically transparent carbon electrode (e.g. a thin film of boron-doped diamond on an optically transparent substrate), a thin solution layer, and an optical window mounted opposite the optically transparent carbon electrode such that transmission spectroscopy can be performed on species within the solution. The optically transparent carbon electrode is used to oxidize and reduce species in the solution. In situ IR and UV-visible spectroscopy is performed through the optically transparent carbon electrode to analyse dissolved species in the solution. Dissolved species which have different IR and UV-visible spectra in different oxidation states can be analysed. Although boron-doped diamond material is opaque at high boron concentrations, thin films of such material have a reasonable optical transparency. It is described that the ability to cross-correlate electrochemical and optical data may provide new insights into the mechanistic aspects of a wide variety of electrochemical phenomena including structure-function relationships of redox-active proteins and enzymes, studies of molecular absorption processes, and as a dual signal transduction method for chemical and biological sensing [see “Measurements: Optically Transparent Carbon Electrodes” Analytical Chemistry, 15-22, 1 Jan. 2008, “Optically Transparent Diamond Electrode for Use in IR Transmission Spectroelectrochemical Measurements” Analytical Chemistry, vol. 79, no. 19, Oct. 1, 2007, “Spectroelectrochemical responsiveness of a freestanding, boron-doped diamond, optically transparent electrode towards ferrocene” Analytica Chimica Acta 500, 137-144 (2003), and “Optical and Electrochemical Properties of Optically Transparent, Boron-Doped Diamond Thin Films Deposited on Quartz” Analytical Chemistry, vol. 74, no. 23, 1 Dec. 2002]. Zhang et al. have also reported the use of an optically transparent boron-doped diamond thin film electrode for performing combined electrochemistry-transmission spectroscopy analysis [see “A novel boron-doped diamond-ciated platinum mesh electrode for spectroelectrochemistry” Journal of Electroanalytical Chemistry 603. 135-141 (2007)].
As an alternative to analysing chemical species while in solution as described above, one useful electro-chemical analysis technique involves applying a suitable voltage to a sensing electrode to electro-deposit chemical species out of solution onto the sensing electrode and then change the voltage to strip the species from the electrode. Different species strip from the electrode at different voltages. Measurement of electric current during stripping generates a series of peaks associated with different species stripping from the sensing electrode at different voltages. Such a stripping voltammetry technique can be used to analyse heavy metal content.
The use of a boron-doped diamond sensor in a stripping voltammetry technique has been described in U.S. Pat. No. 7,883,617B2 (University of Keio). Jones and Compton also describe the use of a boron-doped diamond sensor in stripping voltammetry techniques [see “Stripping Analysis using Boron-Doped Diamond Electrodes” Current Analytical Chemistry, 4, 170-176 (2008)]. This paper includes a review which covers work on a wide range of analytical applications including trace toxic metal measurement and enhancement techniques for stripping voltammetry at boron-doped diamond electrodes including the use of power ultrasound, microwave radiation, lasers and microelectrode arrays. In the described applications a boron-doped diamond material is used for the working/sensing electrode in combination with standard counter and reference electrodes.
McGraw and Swain also describe using stripping voltammetry to analysis metal ions in solution using an electrochemical cell comprising a boron-doped diamond working electrode in combination with standard counter and reference electrodes (a carbon rod counter electrode and a silver/silver chloride reference electrode). It is concluded that boron-doped diamond is a viable alternative to Hg for the anodic stripping voltammetry determination of common metal ion contaminants [see “A comparison of boron-doped diamond thin-film and Hg-coated glassy carbon electrodes for anodic stripping voltammetric determination of heavy metal ions in aqueous media” Analytica Chimica Acta 575, 180-189 (2006)].
In addition to the stripping voltammetry techniques described above, it is also known to use spectroscopic techniques for analysing electro-deposited films. For example, Peeters et al describe the use of cyclic voltammetry to electrochemically deposit cobalt and copper species onto a gold electrode using a three electrode cell comprising a saturated calomel reference electrode, a carbon counter electrode, and a gold working electrode. The gold electrodes comprising electrochemically deposited cobalt and copper species were subsequently transferred to a synchrotron radiation X-ray fluorescence (SR-XRF) facility for SR-XRF analysis to determine the heterogeneity of the deposited layers and the concentrations of Co and Cu. A comparison of SR-XRF results with electrochemical data was used to investigate the mechanism of thin film growth of the cobalt and copper containing species [see “Quantitative synchrotron micro-XRF study of CoTSPc and CuTSPc thin-films deposited on gold by cyclic voltammetry” Journal of Analytical Atomic Spectrometry, 22, 493-501 (2007)].
Ritschel et al. describe electrodeposition of heavy metal species onto a niobium cathode. The niobium cathode comprising the electrodeposited heavy metal species is then transferred to a total reflection X-ray fluorescence (TXRF) spectrometer for TXRF analysis [see “An electrochemical enrichment procedure for the determination of heavy metals by total-reflection X-ray fluorescence spectroscopy” Spectrochimica Acta Part B, 54, 1449-1454 (1999)].
Alov et al. describe electrodeposition of heavy metal species onto a glass-ceramic carbon working electrode. A standard silver chloride reference electrode and a platinum counter electrode were used in the electrochemical cell. The glass-ceramic carbon working electrode comprising the electrodeposited heavy metal species is then transferred to a total reflection X-ray fluorescence (TXRF) spectrometer for TXRF analysis [see “Total-reflection X-ray fluorescence study of electrochemical deposition of metals on a glass-ceramic carbon electrode surface” Spectrochimica Acta Part B, 56, 2117-2126 (2001) and “Formation of binary and ternary metal deposits on glass-ceramic carbon electrode surfaces: electron-probe X-ray microanalysis, total-reflection X-ray fluorescence analysis, X-ray photoelectron spectroscopy and scanning electron microscopy study” Spectrochimica Acta Part B, 58, 735-740 (2003)].
The present inventors have identified a number of potential problems with the aforementioned techniques. For example, while Swain et al. and Zhang et al. have described the use of in-situ spectroscopic techniques through a transparent electrode in an electrochemical sensor to generate spectroscopic data which is complimentary to voltammetry data, the transmission IR and UV-visible spectroscopy techniques described therein are only suitable for analysis of chemical species in solution. They are not suitable for analysing species such as heavy metals electro-deposited on an electrode. Furthermore, as the species are not concentrated by electro-deposition onto an electrode surface then low concentrations of species in solution may be below the detection limit for certain spectroscopic techniques. Further still, such spectroscopic techniques only give information about chemical species in the bulk solution and do not give information about the surface of the sensor to establish, for example, when the surface of an electrode is clean or when minerals or amalgams form on an electrode surface.
In contrast, prior art stripping voltammetry techniques on diamond electrodes are advantageous for analysing species such as heavy metals which can be electro-deposited from solution as described by Jones, Compton, McGraw and Swain. However, species discrimination in multi-metal solutions can be a problem using such techniques since the peak positions can be overlapping in stripping voltammetry data. Furthermore, the use of standard reference and counter electrodes in such arrangements means that the electrochemical sensor is not robust to harsh chemical and physical environments, even if the diamond sensing electrode is robust to such conditions.
The problem of overlapping peaks in stripping voltammetry data can potentially be solved by applying the teachings of Peeters et al., Ritschel et al., and Alov et al. These groups have suggested electro-depositing films onto gold, niobium or glass-ceramic carbon working electrodes and then extracting the electrodes from the electro-deposition apparatus and transferring the coated electrodes to a suitable device for further analysis including, for example, electron-probe X-ray microanalysis, total-reflection X-ray fluorescence analysis, X-ray photoelectron spectroscopy and scanning electron microscopy. However, this technique requires the provision of multiple devices and the extraction of coated electrode components for subsequent analysis which may not be possible for field analysis and/or in remote sensing environments, e.g. down an oil well. Furthermore, the electrodes, particularly gold, can interfere with x-ray analysis techniques such as X-ray fluorescence analysis.
Further still, the electro-deposition and electrochemical sensor apparatus described in the aforementioned documents use electrodes which are not robust to harsh chemical and physical environments. Even those documents which describe the use of synthetic boron-doped diamond material as a sensing/working electrode include less robust materials for the reference and counter electrodes. This is problematic as synthetic doped diamond material will generally be the material of choice for sensing applications in harsh chemical and/or physical environments. However, while synthetic doped diamond material has been proposed for use as a sensing/working electrode in such applications, a standard reference electrode is still required to provide a constant and fixed reference potential in order to be able to assign peaks in voltammetric data. In this regard, it should be noted that the purpose of a reference electrode is usually to maintain a constant potential with respect to the working electrode. According to the Nernst equation the local concentration of redox active or potential determining ions will determine the reference electrode potential. Thus common reference electrodes such as the “saturated calomel electrode” and the “silver/silver chloride electrode” contain a metal coated in its sparingly soluble chloride salt in contact with a saturated concentration of chloride ions. In this way, the concentration of chloride ions next to the electrode surface is maintained at a fixed value irrespective of the solution conditions in which the electrode is placed. Commercial electrodes typically contain such an electrode housed in a glass body in contact with a solution filled with an excess of potassium chloride, separated from the main solution under test using a frit. For device fabrication this design may not be appropriate and so manufacturers often microfabricate Ag structures which they then chloridise to form a thin silver chloride coating. In solution the silver chloride dissolves to form a layer of chloride ions around the surface which can be approximated as being constant, however this is not as stable as a true reference electrode.
The issues with reference electrodes of the aforementioned type are:    (1) Fouling—if the electrode surface fouls it is problematic to clean the electrode by applying potential cleaning cycles without destroying the chemical identity of the reference electrode.    (2) In corrosive solution conditions, e.g. a high or low pH, again chemical degradation of the electrode means the reference electrode potential changes with time in the same solution.    (3) AgCl is light sensitive and can photodecompose, again affecting the stability of the electrode.
It is an aim of certain embodiments of the present invention to address one or more of the aforementioned problems. In particular, certain embodiments of the present invention aim to provide an electrochemical sensor comprising both a robust sensing/working electrode and a robust reference electrode. In addition, certain embodiments provide an electrochemical sensor for monitoring low concentrations of a plurality of chemical species in complex chemical environments. Advantageous arrangements combine this functionality in a device which is relatively compact and is suitable for use in the field and/or in remote and/or harsh sensing environments such as for oil and gas applications.